The evolution of precision bending

Three decades ago bottoming with penetration, or coining, was the 1004only way to achieve high accuracies on press brakes, and this meant fabricators endured high tooling costs. Over the years precision air bending with CNC hydraulic press brakes using precision-ground tooling evolved to become the dominant forming method in the precision market. However, it took some significant machinery advances to get there.

Precision bending has come a long way in recent decades. What used to be considered “precision” in the 1970s wouldn’t pass muster today. Then a fabricator might have considered ±1 degree a precision bend; today a precision bend is within a half or even a quarter of a degree. Several factors have driven the market toward greater precision, including pressure to reduce the cost to produce a part, as well as robotics and automation. While human welders and assemblers can deal with variability, robots and automation cannot. These technologies need accurate parts to function effectively.

In years past if a fabricator scrapped parts, it could afford to make extra ones. But today, with the cost of metal, employees, medical insurance, and so forth, making extra parts is not an option for a company that wants to be profitable.

Three decades ago bottoming with penetration, or coining, was the only way to achieve such accuracies, and this meant fabricators endured high tooling costs. Over the years precision air bending with CNC hydraulic press brakes using precision-ground tooling evolved to become the dominant forming method in the precision market. However, it took some significant machinery advances to get there.

The Basic Terms

Bottoming on the press brake occurs when there is no air between the punch, the material being formed, and the die.
While press brake technology has progressed over the years, the basic press brake process terms haven’t changed.

Bottoming. This is the point during forming at which there is no air between the punch, material being formed, and the die (see Figure 1). The tooling is literally “bottomed out.” If the tooling has a 90-degree included angle, the material will be bent to 90 degrees at the bottom. However, when the ram goes up and the load is removed, the angle will open to something greater than 90 degrees. Bottoming as defined will not produce an accurate bend.

The problem is that the material started out at 180 degrees, and although it was formed to 90 degrees, it has a memory of 180 degrees and desires to get back to it. Once the load is removed, the material will try to spring back to its original shape or angle. The amount of springback will vary according to the material’s tensile strength, yield point, and hardness.

One common way to compensate for the springback is to overbend the material by the amount of the springback. This appears to be a simple solution: Determine the springback and then purchase the correct tools. However, the springback is unpredictable because of a number of variables, not least of which is the lack of homogeneity in the material. True bottoming is not reliable or predictable.

Bottoming with penetration, or coining, helps compensate for springback.
Bottoming With Penetration (Coining). Among bottoming techniques, the only real way to compensate for springback and make an accurate bend is to form the material to the desired angle and then push the nose of the punch into the material. Called bottoming with penetration, or coining—derived from the process of coin-making—the process causes material to flow under heavy pressure (see Figure 2). The nose of the punch penetrates the material, and a plastic flow of material occurs. When it stops flowing, it solidifies and holds the material in the desired position, nullifying the effects of springback.

Unfortunately, the process is extremely hard on tools, and it requires very high tonnage. Bottoming with penetration can require tonnages up to 10 times that required for air bending.

General-purpose Air Bending. The term air bending is derived from the forming method in which the bend is made in air (see Figure 3). In air bending’s truest form, the material touches the tools at only three points (see Figure 4).

Air bending on general-purpose press brakes most often is used for forming thick gauges (10 gauge and thicker) when angle variations of 1 to 2 degrees are satisfactory. It is also commonly used in HVAC shops and general sheet metal shops that do not require accurate bends.

During air bending, the bend is made literally “in the air.”
Precision Air Bending. In precision air bending, the punch is positioned to within ±0.0004 inch or less of the die. Today the only way to achieve this is with precision-ground tooling and a CNC press brake with the required ram repeatability (see sidebar, Precision Air Bending and Ram Repeatability). With precision-ground tooling, a CNC press brake can produce bends to within ±1/2-degree accuracy by precisely controlling the amount of hydraulic oil that goes into the cylinders with either a proportional valve or a servo valve. Each cylinder fill system (the Y1 and Y2 axis) must have its oil flow independently controlled.

Although both precision air bending and bottoming with penetration are still used to attain accurate bends, precision air bending has gained significant ground in recent years, for obvious reasons. With precision air bending, the accuracy of the machine determines the accuracy of the bend, and when compared to bottoming with penetration, tonnage requirements to get that accuracy decrease, as do tooling costs.

Precision With a Mechanical Press

In the truest form of air bending, the material touches the tools at only three points.
Over the past three decades, as press brake technology evolved, precision bending evolved with it. In the early 1970s, bottoming with penetration with a mechanical press brake was the only way to attain a bend ±1 degree. These brakes were the simplest, most bulletproof machines on the market. True, hydraulic presses had emerged, but their rams could repeat to only ±0.004 in. or more, too inaccurate for precision bending.

Think of the mechanical press brake’s motion as it relates to a boxer’s arm throwing a punch. If the arm extends only partway, bent at the elbow, someone can easily move it. But if the arm stretches all the way out, with the elbow locked, it’s incredibly hard for someone to push that fist back and unlock that elbow—that’s the leverage effect. When fully extended, that fist can plow forward with tremendous force.

The same goes for a mechanical press brake. Consider a 90-ton machine bottoming with penetration. It produces 90 tons near the bottom of the stroke, and as it fully extends, the punch penetrates the metal a few thousandths of an inch, building the tonnage and often producing 50 percent more power; so, a 90-ton press brake is able to exert 135 tons.

For this reason, designers built the frames and bearings of these machines to withstand that extra tonnage. The accuracy came with the ram fully extended, producing a “rigid stack” of elements, from the crankshaft to the eccentric, to the screw, to the ram, to the punch, to the material, to the die—as rigid as a boxer’s arm extended.

Mechanical presses required tremendous skill. With no hydraulics or CNC to assist, an operator learned to slip the machine’s clutch with precision. It was almost an art. If he improperly slipped the clutch, he could lower the ram too fast, which would create a hazardous condition for him, as well as a bad part. If he allowed that ram to descend too far, he could inflict major machine damage, ruining the bearings and screws, and perhaps even cracking a side plate. At the same time, these brakes had planed tooling, which meant the tip-to-shoulder height on one end of the tool may have been a few thousandths inch longer than the other end. Operators had to adjust the ram level just right to attain a precise bend and often had to shim sections of the die.

Hydraulics Take Center Stage
Hydromechanical presses were the next step up in accuracy. Introduced during the late 1970s and early 1980s, these represented the first systems using hydraulics that could attain ram repeatability of ±0.001 in. One design introduced in the 1970s replaced the clutch and brake with hydraulic cylinders. This design eliminated the need to have an experienced operator because there was no need to slip a clutch. It had excellent coining capability, with the leverage effect generating up to 50 percent more tonnage at the bottom. But it still did not improve the ram repeatability.

Another design employed a rocker arm, much like a seesaw, with the hydraulic cylinder on one end and the ram on the other. Placing the cylinder three times farther from the pivot point as the ram produced a multiplier effect. To produce 90 tons of bending force, the cylinder had to produce only 30 tons of force. More important, if the cylinder could stop within 0.003 in., then the ram could stop within 0.001 in. This design was the first real breakthrough in increasing the accuracy of a hydraulic press brake. The hydraulic motors gave greater control over the ram, while the mechanical linkages provided the accuracy. Other hydromechanical designs followed, and the design is still very popular today due to its simplicity and price.

By the early 1980s, a mechanical brake bottoming with penetration and air bending with a hydromechanical machine, while producing accurate bends for the time, still could achieve only ±0.001-in. ram repeatability. Servo valves broke that repeatability barrier. On a hydraulic machine, these valves could accurately meter the flow of the oil to the hydraulic cylinder and self-adjust. It was as if machine-makers gave these hydraulic valves a brain. At first these valves were very expensive and sensitive to dirt and oil. Nevertheless, finally a hydraulic system—a more controllable, safer machine—could perform what is considered today precision air bending, with ±0.0004-in. or better ram repeatability.

What came next—and what ultimately pushed precision bending to where it stands today—is the advanced computer numerical control. With better controls, manufacturers transferred the brain from the valves on the cylinder to the control. Proportional valves replaced servo valves. With their continual rise in sophistication and processing power, high-end CNCs can monitor to minutia, measuring down to microns.

Today’s precision brakes offer intelligent monitoring to account for springback, deflection, and material variances. Some techniques rely on bending databases that theoretically predict what is going to happen and adjust before the bending takes place. Most offer devices that allow fabricators to form, measure, and then adjust during the bending process, internal to the press brake (using pressure sensors) or external (with laser tracking devices).

Servo- or electrohydraulic systems and other hybrids have emerged for precision work. Some manufacturers offer all-electric servo brakes that do away with expensive-to-maintain and complex hydraulics and, at the same time, keep their control and accuracy.

The Importance of Tooling
But no matter how accurate the machine, it would do nothing to improve accuracy without accurate tooling. Ground tooling, with tolerances held to ±0.0008 in., remains a requirement for precision air bending on a press brake. Not only does the tooling produce high accuracy, but technologies such as hydraulic clamping with push-button location ease setup, which is particularly valuable for a low-skilled work force.

Balancing Technology and Skill
Historically, precision bending has required three principal elements: operator skill, ram repeatability, and a control that can measure that repeatability. Today the last two have made it possible to bring a relatively low-skilled operator up to speed in precision bending very quickly.

But no matter the technology, metal fabricators must, as always, fit the technology with their business needs. For example, if a shop places an angle-measuring device on a press brake, it may enhance predictable quality, but at what cost? Depending on the technology, such measuring devices may increase ram dwell times; the operation takes longer and, hence, costs more.

Consider a lighting-fixture application using a high-end press brake with real-time measuring and a lower-end brake, both of which can produce the bend within tolerances. A test may reveal that a part can be produced on the lower-end brake in 45 seconds. Because of its dwelling ram, the higher-end brake takes 60 seconds. That’s significantly slower. True, the lower-end brake may require more skill for setup and operation—which may be a real hindrance, especially if a shop has a hard time finding skilled people—but if shop floor employees have basic skills, the lower-end technology may be a good fit.

Ultimately, it’s about attaining a predictable quality at a predictable cost that fits the needs of the markets a shop serves. Throughout the decades, that fact has never changed.

Variables Affecting Precision Air Bending
The accuracy of the final precision air bend on a hydraulic press brake is a function of the following variables:

Machine Variables

Ram repeatability of the press brake
Deflection of the bed and ram
Deflection of the side housings
Response time of the filling system and valves
Ambient and oil temperatures
Time the material is held under load
Tooling Variables

Dimensional tolerance of the punch, die, and die holder
Proper seating and alignment of the tooling
Wear of the punch and die
Material Variables

Homogeneity of the material, particularly the yield strength
Thickness of the material
Grain direction of the material during forming
Material protective coatings
Surface hardness
Precision Air Bending and Ram Repeatability

Ram positioning accuracy affects accurate forming on a press brake. Source: Accurpress America Inc., Rapid City, S.D.
In precision air bending, the exactness of the angle formed is determined by how accurately the punch is positioned relative to the die and how wide the die opening is. If either one of these varies, so does the angle.

The quality of tooling and the die holder plays a role in the proper positioning of the punch relative to the die. If twist is present or the tool’s critical dimensions vary over their length, it will be difficult to make accurate bends. Ground or precision-machined tooling should be considered.

The most important variable is the press brake’s ability to position the punch relative to the die and to repeat—that is, its ram repeatability. Assume a press brake has a ram positioning accuracy of ±0.001 inch, and two forming jobs require tolerances of ±1-degree angle variation. One job forms 10-gauge mild steel and the other 18-ga. mild steel. As a rule of thumb for determining the correct die opening for a 90-degree air bend in mild steel with an inside radius approximately equal to the material thickness, the die opening should be eight times the material thickness. A 10-ga. workpiece requires a 11/8-in. die opening; 18 ga. requires a 3/8-in. die opening.

The figure shows that with a 11/8-in. die opening, ±1-degree accuracy in 10-ga. material can be obtained with a ram positioning accuracy of ±0.001 in., because the angle variation is ±0.3804 degree. The figure also shows that with a 3/8-in. die opening, ±1-degree accuracy in 18-ga. material cannot be obtained with a ram positioning accuracy of ±0.001 in., because the angle variation is ±1.1526 degrees.

Therefore, with a ram positioning accuracy of ±0.001 in., precision air bending can be used for the 10-ga. job, but not for the 18-ga. job.

By Joseph Altieri
June 17, 2008


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